Method of producing a grinding medium

ABSTRACT

A method of producing a grinding medium on the basis of alpha-alumina and at least one alumina carbide selected from the group consisting of Al 2  OC and Al 4  O 4  C comprises the steps of melting a mixture of alumina or a material rich in alumina with a carbon-containing reducing agent to obtain a melt, rapidly cooling the melt to obtain a solidified body, breaking the solidified body into abrasive grains, and subjecting the abrasive grains to a heat treatment at a temperature of 500° C. and 1500° C. for a period of three minutes to 24 hours.

The present invention relates to a method of producing a grinding medium on the basis of alpha-alumina and at least one aluminum carbide, optionally comprising at least one additive.

U.S. Pat. No. 4,341,533 discloses a grinding medium comprised of a combination of a crystalline phase of aluminum oxide and at least one aluminum carbide selected from the group consisting of Al₂ OC and Al₄ O₄ C, wherein the component proportions are such that, in the phase diagram, Al₂ O₃ +Al₄ C₃ corresponds to a mol fraction of Al₄ C₃ between 0.02 and 0.20. The molten mixture is cooled at a controlled speed of 10° C. to more than 100° C. per minute. The cooled melt is then broken up into grains in a conventional manner, classified and used for abrading materials.

It has now been found that the proportion of Al₄ C₃ in the end product increases considerably with an increasing content of carbon in the melt if the mixture of alumina or alumina-rich materials with a carbon-containing reduction agent is molten and then rapidly cooled. Since Al₄ C₃ is decomposed in a humid ambiance and by heat during grinding, the efficiency of the grinding medium is considerably reduced by the presence of Al₄ C₃.

It is an object of this invention to avoid this disadvantage by melting a mixture of a raw material selected from the group consisting of alumina and a material rich in alumina with a carbon-containing reducing agent to obtain a melt, rapidly cooling the melt to obtain a solidified body, breaking the solidified body into abrasive grains, and subjecting the abrasive grains to a heat treatment at a temperature of between 500° C. and 1500° C. for a period of three minutes to 24 hours. The mixture may further comprise a metallic reducing agent, preferably magnesium or aluminum. At least one additive, for example selected from the group consisting of magnesium, calcium, zirconium, titanium, silicon, chromium and a rare earth may be added to the mixture, preferably in an amount of 0.1% to 10%, by weight. The grinding grains may be surface-coated before or after the heat treatment with at least one substance selected from the group consisting of silicates, pulverulent corundum, pigments and iron oxides.

The controlled heat treatment has the purpose of removing the unwanted Al₄ C₃ and to change the structure of the grinding material into a special metastable equilibrium to relieve the internal tensions and lattice deformations (the rapid cooling of the melt resulting in an extreme unbalance in the structure.

The rapid cooling of the melt, involving a cooling time of more than 100° C./min., is absolutely necessary since it assures the formation of a suitable microcrystalline structure required for excellent grinding efficiency, in addition to the composition of the structure. A slower cooling of the melt in the range of 50° C./min., for example, produces a grinding medium containing alpha-alumina and the two oxide carbide phases Al₂ OC and Al₄ O₄ C but the grinding efficiency is unsatisfactory because of the coarse crystalline structure of the product.

The rapid cooling of the melt may be accomplished by casting it in a mold with defined plate distances, by casting on a metallic or non-metallic shaped body or in a salt or metal melt.

Too long an annealing or an annealing process at excessively high temperatures leads to a substantial equilibrium in the structure, i.e. to the disappearance of the Al₂ OC-phase, which has a negative effect on the grinding efficiency, as the following examples will clearly show.

The properties of the grinding medium obtained by the process of the invention have been further improved by the addition of the above-mentioned additives in the preferred indicated amounts.

It has furthermore been found that optimal grinding efficiencies are obtained if the grinding medium contains Al₂ OC and Al₄ O₄ C, the proportion of the Al₂ OC-phase preferably exceeding that of the Al₄ O₄ C-phase.

The carbon-containing reducing agent may be selected from the group consisting of coke, carbon black, graphite, anthrazite and amorphous carbon, or it may be a carbide, such as Al₄ C₃ added, for example, to obtain an amount of less than 5%, preferably less than 0.5%, by weight, in the end product.

The indicated surface coatings, which are applied before or after the annealing heat treatment, depending on the coating substance, further improve the outstanding qualities of the grinding medium of the present invention. The purpose of the coating is the creation of an enlarged grain surface, which decisively improves the bonding of the grinding medium to the grinding tool.

The grain tenacity (grain decomposition) and the grinding properties of the grinding medium produced according to this invention were determined on the basis of grain P36 FEPA-norm (Federation Europeene des Frabricants de Produits Abrasifs) to determine the grinding efficiency.

To determine the grinding efficiency, grain fraction 36 was bonded with a synthetic resin adhesive to a substrate web. The grinding efficiency (material removal per unit of time) was then determined by grinding a carbon C45 steel shaft under constant pressure with a band grinding machine, as compared to grinding with a zirconium-containing corundum of eutectic composition whose melt had been cooled under the same conditions. An index of 100 was arbitrarily selected for the grinding efficiency of the Zr-containing grinding medium produced by casting the melt in an iron mold having a plate distance of 5 to 7 mm.

The grain tenacity (grain decomposition) was determined by the following method:

A screwed-together steel cylinder containing 12 steel balls having a diameter of 19 mm was filled with 50 grams of grain fraction 36 having a defined mesh size. The closed cylinder was then turned for 10 minutes at a constant rotational speed on a roller frame unit and the resultant grains were then subjected to a new mesh size analysis. The grain decomposition is obtained by the proportion of the average grain size before and after milling, according to the formula

    % Grain decomposition=100.average grain size after milling/average grain size before milling

Since the grain decomposition depends not only on the specific material properties but also in the shape of the grains, the resultant values were corrected by a grain shape factor.

The invention will now be further explained in connection with the following specific examples:

EXAMPLE 1 (COMPARATIVE EXAMPLE)

A mixture of 150 kg of calcined argillaceous earth (pure clay) containing 0.35%, by weight, of Na₂ O and 0.6 kg of milled coke (C) was melted in an electric arc furnace. The melt was then cast in a mold with a plate distance of 5-7 mm. The solidified plate-shaped body contained 0.045%, by weight, of C and 0.03%, by weight, of Na₂ O. The material was milled in a commercially available comminuting installation and classified according to FEPA-norms into grinding grains. Grain 36 was subjected for 10 minutes to a heat treatment at 1200° C. to relieve internal tensions and oxidize any carbides that may have been present.

The test results of the thus annealed and non-annealed abrasive grains as well as of the comparative grains of Zr-corundum are given hereinbelow in Table 1.

                  TABLE 1                                                          ______________________________________                                                        Weight %   Efficiency                                                          decomposition                                                                             Index                                                ______________________________________                                         Eutectic Zr-corundum                                                                            38.2         100                                              Non-annealed grain 36 (Ex. 1)                                                                   48.7         49.1                                             Annealed grain 36 (Ex. 1)                                                                       41.5         65.0                                             ______________________________________                                    

The X-ray structural micro-analysis (hereinafter termed RDA) of the material showed that the annealed as well as the non-annealed material consisted exclusively of alpha-alumina, within the established limits of proof.

The following Examples 2-6 proceeded essentially under the same conditions as Example 1. The sole changes were made in the reducing agent and the type of heat treatment used (Example 7).

EXAMPLE 2

The reducing agent was 3.5 kg of milled cokes. The solidified plate-shaped material contained 0.4%, by weight, of C and 0.03%, by weight, of Na₂ O. The RDA showed the material to consist primarily of alpha-alumina, with a small amount of Al₂ OC. No Al₄ C₃ was found.

EXAMPLE 3

Eight kg of milled coke was used as reducing agent. The product was found to contain 1.1%, by weight, of C and less than 0.02%, by weight, of Na₂ O. The RDA showed the material to consist primarily of alpha-alumina and Al₂ OC, with minor amounts of Al₄ O₄ C and Al₄ C₃.

EXAMPLE 4

Ten kg of milled coke was used as reducing agent and the resultant product has a C content of 1.4%, by weight, and an Na₂ O content of less than 0.02%, by weight. The RDA showed the material to consist primarily of alpha-alumina, Al₂ OC and Al₄ O₄ C, the amount of Al₄ C₃ in the material being substantially more than in that of Example 3 and comprising 2-3%, by weight.

EXAMPLE 5

Sixteen kg of milled coke were used as reducing agent and the product contained 2.4%, by weight, of C and less than 0.02%, by weight, of Na₂ O. When the furnace charge was melted, volatilization of aluminum was clearly noticed. In addition, the melt foamed considerably so that the furnace charge had to be bricketted to avoid excessive dusting.

The RDA showed a product consisting of alpha-alumina, Al₂ OC and Al₄ O₄ C, additionally containing about 4-5%, by weight, of Al₄ C₃.

EXAMPLE 6

In this case, too, the furnace charge comprised of 150 kg of calcined argillaceous earth (pure clay) and 25 kg of milled coke (C) was bricketted. When the bricks were melted, a substantial amount of the aluminum was volatilized and, furthermore, the melt foamed considerably. The product cast in the mold contained 4.5%, by weight, of C and the Na₂ O-content was below 0.02%, by weight. The RDA showed a large amount of Al₂ OC and Al₄ O₄ C in addition to alpha-alumina and a 10%, by weight, content of Al₄ C₃.

EXAMPLE 7

The grains obtained by the processes of Examples 2 to 6 and classified were subjected to an annealing heat treatment to remove any Al₄ C₃, to alleviate internal tensions and to adjust to a specific metastable equilibrium in the structure. The annealing temperatures were 1000° C., 1100° C., 1200° C., 1300° C., 1500° C. and 1800° C. and the annealing times were 3, 5, 10, 20, 40, 60, 120 and 240 minutes. Since the heat treatment was designed solely to oxidize the carbide to respective oxide carbides, annealing beyond 10 minutes was effected in a controlled atmosphere (primarily an inert atmosphere). It was desired to keep the carbon content after the heat treatment not substantially lower than before the treatment.

The examination of the products was carried out in the above-indicated manner and the test results are shown in Table 2.

                  TABLE 2                                                          ______________________________________                                         Ex.             Treatment Grain decompo-                                                                           Grinding ef-                               No.  °C. time/min. sition %, by wt.                                                                         ficiency index                             ______________________________________                                         2    not annealed                                                                              --        45.0      49.8                                       "    1000        3        45.0      50.2                                       "               10        41.3      60.2                                       "               20        41.1      65.8                                       "               40        40.8      60.3                                       "               120       42.8      51.2                                       "    1100        3        44.8      54.6                                       "               10        39.8      65.4                                       "               20        42.8      65.1                                       "               40        44.5      68.2                                       "               120       45.8      51.5                                       "               240       46.3      50.8                                       "    1200        5        40.8      58.4                                       "               10        39.3      67.5                                       "               20        39.8      67.3                                       "               40        43.5      58.1                                       "               60        44.9      51.1                                       "               240       48.0      47.1                                       "    1300        3        41.8      59.5                                       "                5        39.9      64.5                                       "               10        41.5      62.7                                       "               20        43.8      59.1                                       "               40        45.5      50.6                                       "               240       48.5      46.5                                       "    1400        3        39.2      63.5                                       "               10        43.5      58.2                                       "               60        48.1      46.5                                       "               240       48.2      46.4                                       2    1500        3        45.3      52.5                                       "                5        44.6      52.5                                       "               20        48.6      44.6                                       "               60        51.2      41.2                                       "               240       51.8      41.0                                       "    1800        3        47.2      50.3                                       "                5        46.5      51.0                                       "               10        50.3      47.6                                       "               60        55.8      40.1                                       "               240       56.1      40.0                                       3    not annealed         41.2      66.3                                       "    1000        3        41.3      67.1                                       "                5        40.8      69.4                                       "               10        39.1      78.2                                       "               15        38.5      95.4                                       "               20        37.8      102.3                                      "               40        37.8      100.1                                      "               60        38.9      91.4                                       "               120       41.5      68.3                                       "               240       42.3      60.5                                       "    1100        5        39.4      81.2                                       "               10        37.8      98.5                                       "               20        37.2      103.2                                      "               40        38.5      95.8                                       "               120       43.5      60.2                                       "    1200        5        37.6      90.3                                       "               10        37.0      100.5                                      "               20        36.8      105.6                                      "               40        38.8      88.3                                       "               120       42.3      61.7                                       "    1300        3        40.5      78.6                                       "                5        36.5      104.3                                      "               10        35.8      106.2                                      "               20        37.3      100.8                                      "               40        39.8      81.4                                       "               120       42.1      60.2                                       3    1500        3        41.6      88.4                                       "                5        40.3      98.1                                       "               10        42.8      87.3                                       "               40        45.8      61.4                                       "               120       52.5      54.3                                       "    1800        3        43.5      67.3                                       "               20        52.5      45.1                                       "               240       58.3      39.5                                       "    not annealed         37.4      95.3                                       4    1000        5        35.4      104.5                                      "               10        34.6      106.3                                      "               20        34.3      108.5                                      "               40        35.8      106.3                                      "               60        38.9      97.2                                       "               240       43.0      71.3                                       "    1100        3        37.0      100.3                                      "                5        34.6      107.3                                      "               10        33.9      118.5                                      "               20        34.3      109.4                                      "               60        45.6      60.3                                       "    1200        3        35.8      102.6                                      "                5        33.9      114.8                                      "               10        32.4      130.3                                      "               20        35.9      104.6                                      "               40        39.9      90.5                                       "               240       56.1      41.3                                       "    1300        3        34.5      108.5                                      "                5        32.6      127.8                                      "               10        32.8      129.3                                      "               20        36.3      100.4                                      "               60        51.9      49.6                                       "    1500        3        36.5      105.2                                      "                5        43.2      78.5                                       "               40        58.6      44.6                                       4    1800        3        44.3      60.5                                       "                5        51.6      52.3                                       "               10        60.4      40.6                                       "               120       60.8      40.3                                       5    not annealed         35.8      105.6                                      "    1000        3        35.2      109.6                                      "                5        34.0      115.5                                      "               10        33.2      118.5                                      "               20        33.0      128.6                                      "               40        34.5      114.0                                      "               60        35.8      108.6                                      "               240       41.5      85.9                                       "    1100        3        35.1      108.2                                      "                5        33.6      118.4                                      "               10        32.9      129.6                                      "               20        32.5      136.3                                      "               40        36.8      105.2                                      "               240       55.6      45.8                                       "    1200        3        35.2      109.5                                      "                5        32.8      132.3                                      "               10        32.4      145.6                                      "               20        34.3      117.6                                      "               40        38.5      100.6                                      "               240       56.3      53.0                                       "    1300        3        34.6      112.5                                      "                5        32.6      140.6                                      "               10        33.8      120.3                                      "               20        37.2      100.5                                      "               120       54.6      51.3                                       "    1500        3        34.8      110.5                                      "                5        40.2      98.3                                       "               20        53.5      55.3                                       "               240       61.5      40.5                                       "    1800        3        48.6      61.3                                       "               40        61.3      39.8                                       "               240       60.3      39.4                                       6    not annealed         37.0      93.2                                       "    1000        3        36.1      104.6                                      "               10        34.5      111.4                                      "               20        33.8      111.0                                      "               40        35.8      105.4                                      "               60        41.3      90.6                                       "               240       48.7      65.3                                       "    1100        3        35.3      108.4                                      "               10        33.4      119.5                                      "               20        35.6      108.3                                      "               40        38.2      101.4                                      "               240       50.2      55.0                                       "    1200        3        34.8      112.3                                      "                5        33.5      118.4                                      "               10        33.2      118.6                                      "               20        38.5      103.6                                      "               240       55.4      48.3                                       "    1300        3        34.2      117.3                                      "                5        34.0      119.2                                      "               10        35.4      109.6                                      "               20        40.3      98.4                                       "               60        48.3      61.4                                       "    1500        3        39.2      100.8                                      "                5        40.0      96.4                                       "               40        60.5      44.6                                       "               240       71.0      28.0                                       "    1800        3        51.5      50.6                                       "               40        68.3      38.7                                       "               240       72.3      28.3                                       ______________________________________                                    

The RDA of the annealed products showed that no Al₄ C₃ could be found after, for example, an annealing heat treatment of 10 minutes at a temperature of 1200° C. If the temperature was lower, the heat treatment time had to be correspondingly extended while it was shortened as the temperature was increased. At the same time that the carbides were oxidized, the proportion of the Al₂ OC-phase was changed, i.e. it was somewhat reduced in favor of the Al₄ O₄ C-phase.

EXAMPLE 8 (COMPARATIVE EXAMPLE)

A mixture comprised of 150 kg of calcined argillaceous earth (pure clay) containing 0.35%, by weight, of Na₂ O, 1.5 kg of MgO and 0.6 kg of milled coke was melted in an electric arc furnace in the same manner as in Example 1. The melt was cast in a mold with a plate distance of 5-7 mm. The solidified material had a carbon content of 0.05%, by weight, an MgO-content of 1.03%, by weight, and an Na₂ O-content of 0.03%, by weight. It was milled, classified and annealed at a temperature of 1200° C. for 10 minutes. The examination of the grain properties of grain P 36 (FEPA-norma) was carried out in the above-described manner and the test results are given in Table 3.

                  TABLE 3                                                          ______________________________________                                                                   Grinding ef-                                                   Wt. % grain decomposition                                                                      ficiency index                                       ______________________________________                                         Grain P 36 (Ex. 8)                                                                         45.8              55.8                                             not annealed                                                                   Grain P 36 (Ex. 8)                                                                         40.2              71.3                                             annealed                                                                       ______________________________________                                    

The RDA showed the material to consist of alpha-alumina and MgO.13Al₂ O₃. No significant phase changes occurred in the heat treatment.

The treatment conditions in the following Examples 9 to 15 were essentially the same as in Examples 1-8. The changes relate to the reducing agent, the MgO-additive and the manner of the annealing heat treatment, which is described in Example 16.

EXAMPLE 9

A mixture of 150 kg of calcined argillaceous earth (pure clay), 1.5 kg of MgO and 6.0 kg of milled coke (C) was melted in an electric arc furnace. The solidified product contained, by weight, 0.8% C, 1.08% MgO and less than 0.02% Mx₂ O. The RDA showed the material to be comprised of alpha-alumina and the compounds MgO.13Al₂ O₃, Mg-O-Al and Al₂ OC. The Al₄ C₃ -content was below provable limits.

EXAMPLE 10

One hundred and fifty kilogram of calcined argillaceous earth (pure clay), 1.5 kg of MgO and 13 kg of milled coke were thoroughly mixed, bricketted and melted in an electric arc furnace. The product contained, by weight, 2.0% C, 1.1% MgO and less than 0.02% Na₂ O. The RDA showed the product to be comprised primarily of alpha-alumina, MgO.13Al₂ O₃, Al₂ OC, Al₄ O₄ C and about 3-4%, by weight, of Al₄ C₃.

EXAMPLE 11

A briketted mixture of 150 kg of calcined argillaceous earth (pure clay), 1.5 kg of MgO and 20 kg of milled coke (C) was melted in an electric arc furnace. The product solidified in a mold contained, by weight, 3.1% C, 1.1% MgO and less than 0.02% Mx₂ O. The RDA showed the material to be comprised of alpha-alumina and the compounds MgO.13Al₂ O₃, Al₂ OC, Al₄ O₄ C and 8%, by weight, of Al₄ C₃.

EXAMPLE 12

A mixture of 150 kg of calcined argillaceous earth (pure clay), 3.0 kg of MgO and 0.6 kg of milled coke (C) was melted and cast in the manner described in Example 1. The product contained, by weight, 0.05% C, 1.95% MgO and 0.03% Na₂ O. The RDA showed the prevailing phases to consist of alpha-alumina and MgO.13Al₂ O₃, with some Mg-Al-O. No carbides were found.

EXAMPLE 13

One hundred and fifty kilogram of calcined argillaceous earth (pure clay), 3.0 kg of MgO and 13 kg of milled coke were mixed and the mixture was bricketted. After melting and casting in a mold, the product contained, by weight, 1.96% C, 2.04% MgO and less than 0.02% Na₂ O. According to the RDA, the material was comprised of alpha-alumina, MgO.13Al₂ O₃, Al₂ OC, Al₄ O₄ C and 4-5%, by weight, of Al₄ C₃.

EXAMPLE 14

One hundred and fifty kilogram of calcined argillaceous earth (pure clay), 5.4 kg of MgO and 0.6 kg of milled coke were thoroughly mixed and melted in an electric arc furnace. The melt was cast in a mold with a plate distance of 5-7 mm. The solidified product contained, by weight, 0.045% C, 3.48% MgO and 0.04% Na₂ O. The RDA showed the product to be comprised of alpha-alumina, MgO.13Al₂ O₃, a little Mg-Al-O and spinel.

EXAMPLE 15

A mixture of 150 kg of calcined argillaceous earth (pure clay), 5.4 kg of MgO and 13 kg of milled coke (C) was bricketted and melted and cast in the manner described in Example 1. The product contained, by weight, 1.94% C, 3.55% MgO and less than 0.02% Na₂ O. The RDA showed the material to be comprised of alpha-alumina, MgO.13Al₂ O₃, Mg-Al-O and the oxycarbides Al₂ OC and Al₄ O₄ C, in addition to 5%, by weight, of Al₄ C₃.

EXAMPLE 16

Analogous to Example 7, the classified grains obtained in Examples 9 to 15 were subjected to an annealing heat treatment. The materials of Examples 12 and 14 were heated for 10 minutes at a temperature of 1200° C. since they required no oxidation of carbides. All other products were treated at different temperatures for different lengths of time.

The test results derived from grains P 36 (FEPA) of Examples 9-15 are shown in Table 4.

                  TABLE 4                                                          ______________________________________                                         Ex.             Treatment Grain decompo-                                                                           Grinding ef-                               No.  °C. time/min. sition %, by wt.                                                                         ficiency index                             ______________________________________                                          9   not annealed                                                                              --        42.9      65.3                                       "    1000        3        42.8      64.5                                       "                5        39.5      89.3                                       "               10        36.8      109.3                                      "               20        35.2      111.5                                      "               40        34.8      113.2                                      "               60        37.5      105.3                                      "               120       39.6      92.5                                       "               240       40.2      90.6                                       "    1100        3        42.5      65.3                                       "                5        38.6      96.4                                       "               10        35.2      115.6                                      "               20        34.5      120.1                                      "               40        36.3      112.3                                      "               60        38.5      100.3                                      "               240       44.4      68.3                                       "    1200        3        41.6      77.3                                       "                5        37.8      103.2                                      "               10        34.0      125.4                                      "               20        33.3      131.2                                      "               40        38.4      101.6                                      "               60        41.5      81.3                                       "    1300        3        40.2      99.5                                       "                5        35.3      113.2                                      "               10        33.6      129.7                                      "               20        37.5      102.6                                       9   1300       40        40.3      91.5                                       "               60        43.1      70.6                                       "               240       44.6      59.8                                       "    1500        3        38.4      100.5                                      "                5        38.6      100.3                                      "               10        39.3      95.4                                       "               20        41.2      80.3                                       "               60        44.6      60.5                                       "    1800        3        40.3      91.2                                       "                5        41.5      80.4                                       "               10        42.6      64.3                                       "               240       45.3      55.0                                       10   not annealed         37.5      102.3                                      "    1000        3        36.7      109.9                                      "                5        35.2      118.6                                      "               10        33.0      132.6                                      "               20        32.3      150.5                                      "               40        32.5      150.5                                      "               60        34.7      118.6                                      "               120       40.8      100.3                                      "    1100        3        35.0      117.6                                      "                5        33.3      129.4                                      "               10        32.0      151.3                                      "               20        31.0      153.6                                      "               40        34.1      122.5                                      "               60        36.3      113.1                                      "               120       38.3      100.6                                      "    1200        3        35.1      118.3                                      "                5        32.0      154.6                                      "               10        31.4      160.2                                      "               20        31.0      171.3                                      "               40        35.3      120.6                                      "               120       44.6      67.5                                       10   1300        3        34.8      114.6                                      "                5        32.0      151.8                                      "               10        32.3      148.5                                      "               20        32.8      138.4                                      "               40        36.5      110.6                                      "               60        42.6      67.3                                       "    1500        3        34.5      115.3                                      "                5        37.2      108.5                                      "               40        48.3      51.3                                       "               240       54.1      45.8                                       "    1800        3        36.3      111.5                                      "                5        44.3      70.8                                       "               20        50.4      50.6                                       "               120       54.8      45.6                                       11   not annealed         36.4      108.4                                      "    1000        3        36.4      110.6                                      "                5        34.8      120.5                                      "               10        33.2      128.4                                      "               20        33.0      130.6                                      "               40        38.5      105.4                                      "               60        39.8      100.6                                      "               240       41.3      90.2                                       "    1100        3        35.2      117.4                                      "                5        33.0      126.5                                      "               10        32.8      140.8                                      "               20        32.1      145.6                                      "               40        35.8      115.3                                      "               60        39.9      100.3                                      "               240       48.6      50.3                                       "    1200        3        35.3      117.9                                      "                5        32.8      138.8                                      "               10        32.0      159.6                                      "               20        32.3      141.8                                      "               40        39.6      102.5                                      "               60        45.2      80.3                                       11   1300        3        33.8      125.6                                      "                5        32.4      141.5                                      "               10        32.8      137.9                                      "               20        37.0      108.5                                      "               40        43.5      76.2                                       "               120       55.3      48.6                                       "    1500        3        34.2      118.5                                      "                5        38.6      101.2                                      "               20        48.3      50.6                                       "               240       61.3      42.3                                       "    1800        3        41.6      85.7                                       "               10        51.3      50.6                                       "               240       66.8      42.0                                       12   not annealed         43.2      65.8                                       "    1200       10        41.1      68.5                                       13   not annealed         38.7      100.3                                      "    1000        3        38.5      100.4                                      "               10        35.8      116.5                                      "               20        34.3      128.4                                      "               40        34.0      130.0                                      "               60        36.0      110.2                                      "               120       38.2      101.4                                      "    1100        3        36.7      108.4                                      "                5        35.3      119.6                                      "               10        34.7      125.4                                      "               20        33.9      135.6                                      "               40        35.0      121.6                                      "               60        38.4      101.4                                      "               240       50.3      67.6                                       "    1200        3        36.3      112.3                                      "                5        34.5      121.6                                      "               10        33.5      137.6                                      "               20        33.4      145.2                                      "               40        37.8      105.6                                      "               60        40.2      100.4                                      "               240       49.5      54.3                                       13   1300        3        35.0      120.3                                      "                5        33.8      130.2                                      "               10        33.2      141.6                                      "               20        37.6      108.4                                      "               60        47.3      65.2                                       "    1500        3        35.0      117.6                                      "                5        37.6      109.3                                      "               10        46.5      61.3                                       "               40        53.7      50.3                                       "    1800        3        44.3      73.5                                       "               10        49.3      58.6                                       "               60        65.4      40.3                                       14   not annealed         46.8      61.4                                       "    1200       10        43.3      75.6                                       15   not annealed         40.3      90.8                                       "                3        38.7      95.3                                       "               10        36.8      109.8                                      "               20        34.8      115.6                                      "               40        34.5      118.6                                      "               60        38.5      98.3                                       "               240       46.3      63.4                                       "    1100        3        38.1      102.4                                      "                5        36.8      108.9                                      "               10        35.3      118.4                                      "               20        34.0      128.5                                      "               40        36.3      109.4                                      "               60        40.2      93.5                                       "               240       48.5      53.9                                       "    1200        3        35.8      115.9                                      "                5        35.7      120.6                                      "               10        33.5      130.2                                      "               20        33.9      135.6                                      "               40        39.3      90.4                                       "    1300        3        34.0      128.4                                      "                5        33.6      137.4                                      "               10        35.2      121.3                                      "               20        38.4      100.8                                      "               120       54.3      50.0                                       15   1500        3        34.8      115.2                                      "                5        39.2      90.0                                       "               10        44.6      71.5                                       "               40        54.3      50.6                                       "               240       63.4      42.5                                       "    1800        3        46.5      59.3                                       "               10        49.1      55.3                                       "               20        57.3      50.2                                       "               240       67.8      39.5                                       ______________________________________                                    

The RDA of the annealed materials showed that those materials which were sufficiently annealed (for example, for 10 minutes at 1200° C.) contained no traceable amounts of Al₄ C₃. Furthermore, the phase Al₄ O₄ C was more prevalent and the proportion of the Al₂ OC-phase was smaller. As the heat treatment time was lengthened, particularly at higher temperatures (above 1200° C.), any Mg-Al-O-phase present in the product decreased substantially.

EXAMPLE 17

A mixture of 150 kg of calcined argillaceous earth (pure clay) containing 0.35%, by weight, of Na₂ O, 3.1 kg of zirconium oxide and 7 kg of milled coke (C) was melted in the manner described in Example 1 and the melt was cast in a mold with a plate distance of 5-7 mm. The solidified product contained, by weight, 0.91% C, 2.06% ZrO₂ and less than 0.02% Na₂ O. The material was milled and classified, and only the grain tenacity and the grinding efficiency index of grain P 36 (FEPA-norm) was determined before and after heat treatment (10 min./1200° C.). Since the zirconium was present in the product primarily in the form of carbide or oxycarbide and sub-oxide, the heat treatment had to be effected partially in an inert atmosphere to prevent their strong oxidation.

The RDA showed that the product was comprised of alpha-alumina, Al₂ OC and zirconium carbide. No suboxides and Al₄ C₃ could be traced. The heat treatment did not substantially change the structure of the product, except for the appearance of a minor Al₄ O₄ C-phase.

The results are indicated in the following Table 5.

                  TABLE 5                                                          ______________________________________                                                                   Grinding ef-                                                   Wt. % grain decomposition                                                                      ficiency index                                       ______________________________________                                         Grain P 36  38.4              101.6                                            not annealed                                                                   Grain P 36 (Ex. 8)                                                                         36.3              110.4                                            annealed                                                                       ______________________________________                                    

EXAMPLE 18

One hundred and fifty kilogram of calcined argillaceous earth (pure clay), 3 kg of SE-oxide and 7 kg of milled coke were mixed and melted in an electric arc furnace. The melt was cast in a mold with a plate distance of 5-7 mm. The solidified product contained, by weight, 0.88% C, 2.1% SE-oxide and less than 0.02% Na₂ O. It was comminuted, classified and the grain decomposition and the grinding efficiency index was again determined from grain P 36 before and after heat treatment for 10 minutes at 1200° C.

The RDA showed the non-annealed product to be comprised of alpha-alumina, Al₂ OC and a phase which could not be defined specifically. No Al₄ C₃ could be traced. Annealing produced a minor Al₄ O₄ C-phase.

The results are shown in Table 6.

                  TABLE 6                                                          ______________________________________                                                                   Grinding ef-                                                   Wt. % grain decomposition                                                                      ficiency index                                       ______________________________________                                         Grain P 36  39.8               92.5                                            not annealed                                                                   Grain P 36 (Ex. 8)                                                                         35.4              115.6                                            annealed                                                                       ______________________________________                                    

EXAMPLE 19

One hundred and ten kilogram of calcined argillaceous earth (pure clay) containing 0.35%, by weight, of Na₂ O, 1.5 kg of MgO and 28 kg of pulverized Al₄ C₃ were well mixed and melted in an electric arc furnace. The melt was cooled as in Example 1. The product contained, by weight, 0.83% C, 1.1% MgO and less than 0.02% Na₂ O. Again, the resultant grain P 36 was tested for grain decomposition and grinding efficiency index before and after heating for 10 minutes at 1200° C. The RDA showed that the non-annealed grain was comprised of alpha-alumina, MgO.13Al₂ O₃, Mg-Al-O and Al₂ OC. Heating caused the Mg-Al-O-phase to be decreased a little while some Al₄ O₄ C appeared.

Table 7 shows the test results.

                  TABLE 7                                                          ______________________________________                                                                   Grinding ef-                                                   Wt. % grain decomposition                                                                      ficiency index                                       ______________________________________                                         Grain P 36  41.8               75.6                                            not annealed                                                                   Grain P 36 (Ex. 8)                                                                         33.9              127.8                                            annealed                                                                       ______________________________________                                    

EXAMPLE 20

One hundred and thirty-five kilograms of calcined argillaceous earth (pure clay) containing 0.35%, by weight, of Na₂ O, 8 kg of Al-sand, 1.8 kg of Mg-sand and 7 kg of milled coke (C) were mixed, the mixture was melted and rapidly cooled. The product contained, by weight, 2.01% MgO, 1.89% C and less than 0.02% Na₂ O. The product was milled and classified, and the resultant P 36 grain was again tested for grain decomposition and the grinding efficiency index before and after the same heat treatment as above. The RDA showed the non-annealed product to be comprised of alpha-alumina, MgO.13Al₂ O₃, Al₂ OC, Al₄ O₄ C and about 1-2%, by weight, of Al₄ C₃. The latter disappeared during the heat treatment and the proportion of Al₂ OC to Al₄ O₄ C changed in favor of the latter.

Table 8 shows the test results.

                  TABLE 8                                                          ______________________________________                                                                   Grinding ef-                                                   Wt. % grain decomposition                                                                      ficiency index                                       ______________________________________                                         Grain P 36  39.5               95.6                                            not annealed                                                                   Grain P 36 (Ex. 8)                                                                         34.9              120.6                                            annealed                                                                       ______________________________________                                    

EXAMPLE 21

To improve the bonding qualities of grain P 36 obtained in Example 10 in the synthetic resin base on the grinding band, the grains were coated. Various types of coatings were applied, the coating being effected in the following manner:

After the abrasive grains were wetted with an adhesive medium (waterglass, colloidal silica, starch, etc.) in an agitator, the coating itself was applied, care being taken that the grains were completely enveloped in the coating. Subsequently, the coating was sintered to obtain a large surface structure and good adherence. If the sintering of the coating required a temperature of about 1200° C., the heat treatment and the sintering was effected in a single operating stage. If the sintering temperatures were lower, the grains were annealed before the coating.

Different types of coatings are shown in the following Table:

    ______________________________________                                         Adhesive Medium Coating Material                                                                             Sintering °C.                             ______________________________________                                         19.1  Waterglass    Fe.sub.2 O.sub.3                                                                              800                                         19.2  Colloidal silica                                                                             CaO           1200                                         19.3  "             ZrO.sub.2.SiO.sub.2                                                                          1200                                         19.4  Starch        Feldspar      1200                                         ______________________________________                                    

The following Table 9 shows the grain decomposition and grinding efficiency index of the coated abrasive grains produced in the above manner. Any increased grain tenacity results from the sealing of cracks produced by comminution by the penetrating adhesive medium.

                  TABLE 9                                                          ______________________________________                                         Abrasive    Wt. % grain Grinding ef-                                           Medium      decomposition                                                                              ficiency index                                         ______________________________________                                         19.1        32.4        175.6                                                  19.2        30.5        185.3                                                  19.3        30.1        191.1                                                  19.4        32.0        174.8                                                  ______________________________________                                    

The results of the Examples given hereinabove show clearly that the grinding medium produced by the method of the present invention has extraordinary grain tenacity and an excellent grinding efficiency. These properties are expressed in a low grain decomposition and a high grinding efficiency index. 

What is claimed is:
 1. A method of producing a grinding medium consisting essentially of alpha-alumina, Al₂ OC and Al₄ O₄ C, which comprises the steps of(a) melting a mixture of a raw material selected from the group consisting of alumina and a material rich in alumina with a carbon-containing reducing agent to obtain a melt, (b) cooling the melt at a speed of more than 100° C. per minute to obtain a solidified body, (c) breaking the solidified body into abrasive grains, and (d) subjecting the abrasive grains to a heat treatment at a temperature of between 500° C. and 1500° C. for a period of three minutes to 24 hours until their Al₄ C₃ -content is less than 5%, by weight, said heat treatment being limited to a maximum period of ten minutes when said temperature is 1500° C.
 2. The method of claim 1, wherein the mixture further comprises a metallic reducing agent.
 3. The method of claim 2, wherein the metallic reducing agent is selected from the group consisting of magnesium and aluminum.
 4. The method of claim 1, further comprising the step of adding to the mixture 0.1-10%, by weight, of at least one additive selected from the group consisting of magnesium, calcium, zirconium, titanium, silicon, chromium and a rare earth.
 5. The method of claim 1, wherein the abrasive grains are surface-coated with at least one substance selected from the group consisting of silicates, pulverulent corundum, and iron oxides.
 6. The method of claim 5, wherein the abrasive grains are surface-coated before the heat treatment.
 7. The method of claim 5, wherein the abrasive grains are surface-coated after the heat treatment.
 8. The method of claim 1, wherein the abrasive grains are subjected to the heat treatment at a temperature between 1000° C. and 1300° C. for a period of 5 to 60 minutes.
 9. The method of claim 1, wherein the carbon-containing reducing agent is selected from the group consisting of coke, carbon black, graphite, anthracite and amorphous carbon.
 10. The method of claim 1, wherein the carbon-containing reducing agent is a carbide.
 11. The method of claim 10, wherein the carbide is Al₄ C₃.
 12. The method of claim 1, wherein the Al₄ C₃ -content is less than 0.5%, by weight.
 13. The method of claim 1, wherein the carbon content of the mixture is between 0.1 and 6.6%, by weight. 